Phase separation of BuGZ promotes Aurora A activation and spindle assembly

Liquid-liquid phase separation of microtubule-binding proteins in the regulation of spindle assembly

Cell division is a highly regulated process essential for the accurate segregation of chromosomes. Central to this process is the assembly of a bipolar mitotic spindle, a highly dynamic microtubule (MT)-based structure responsible for chromosome movement. The nucleation and dynamics of MTs are intricately regulated by MT-binding proteins. Over the recent years, various MT-binding proteins have been reported to undergo liquid-liquid phase separation, forming either single- or multi-component condensates on MTs. Herein, we provide a comprehensive summary of the phase separation characteristics of these proteins. We underscore their critical roles in MT nucleation, spindle assembly and kinetochore-MT attachment during the cell division process. Furthermore, we discuss the current challenges and various remaining unsolved problems, highlights the ongoing research efforts aimed at a deeper understanding of the role of the phase separation process during spindle assembly and orientation. Our review aims to contribute to the collective knowledge in this area and stimulate further investigations that will enhance our comprehension of the intricate mechanisms governing cell division.

Aurora-A condensation mediated by BuGZ aids its mitotic centrosome functions

Centrosomes composed of centrioles and the pericentriolar material (PCM), serve as the platform for microtubule polymerization during mitosis. Despite some centriole and PCM proteins have been reported to utilize liquid-liquid phase separation (LLPS) to perform their mitotic functions, whether and how centrosomal kinases exert the coacervation in mitosis is still unknown. Here we reveal that Aurora-A, one key centrosomal kinase in regulating centrosome formation and functions, undergoes phase separation in vitro or in centrosomes from prophase, mediated by the conserved positive-charged residues inside its intrinsic disordered region (IDR) and the intramolecular interaction between its N- and C-terminus. Aurora-A condensation affects centrosome maturation, separation, initial spindle formation from the spindle pole and its kinase activity. Moreover, BuGZ interacts with Aurora-A to enhance its LLPS and centrosome functions. Thus, we propose that Aurora-A collaborates with BuGZ to exhibit the property of LLPS in centrosomes to control its centrosome-dependent functions from prophase.

Recent Updates on Oncogenic Signaling of Aurora Kinases in Chemosensitive, Chemoresistant Cancers: Novel Medicinal Chemistry Approaches for Targeting Aurora Kinases

The Aurora Kinase family (AKI) is composed of serine-threonine protein kinases involved in the modulation of the cell cycle and mitosis. These kinases are required for regulating the adherence of hereditary-related data. Members of this family can be categorized into aurora kinase A (Ark-A), aurora kinase B (Ark-B), and aurora kinase C (Ark-C), consisting of highly conserved threonine protein kinases. These kinases can modulate cell processes such as spindle assembly, checkpoint pathway, and cytokinesis during cell division. The main aim of this review is to explore recent updates on the oncogenic signaling of aurora kinases in chemosensitive/chemoresistant cancers and to explore the various medicinal chemistry approaches to target these kinases. We searched Pubmed, Scopus, NLM, Pubchem, and Relemed to obtain information pertinent to the updated signaling role of aurora kinases and medicinal chemistry approaches and discussed the recently updated roles of each aurora kinases and their downstream signaling cascades in the progression of several chemosensitive/chemoresistant cancers; subsequently, we discussed the natural products (scoulerine, Corynoline, Hesperidin Jadomycin-B, fisetin), and synthetic, medicinal chemistry molecules as aurora kinase inhibitors (AKIs). Several natural products' efficacy was explained as AKIs in chemosensitization and chemoresistant cancers. For instance, novel triazole molecules have been used against gastric cancer, whereas cyanopyridines are used against colorectal cancer and trifluoroacetate derivatives could be used for esophageal cancer. Furthermore, quinolone hydrazine derivatives can be used to target breast cancer and cervical cancer. In contrast, the indole derivatives can be preferred to target oral cancer whereas thiosemicarbazone-indole could be used against prostate cancer, as reported in an earlier investigation against cancerous cells. Moreover, these chemical derivatives can be examined as AKIs through preclinical studies. In addition, the synthesis of novel AKIs through these medicinal chemistry substrates in the laboratory using in silico and synthetic routes could be beneficial to develop prospective novel AKIs to target chemoresistant cancers. This study is beneficial to oncologists, chemists, and medicinal chemists to explore novel chemical moiety synthesis to target specifically the peptide sequences of aurora kinases in several chemoresistant cancer cell types.

SKAP interacts with Aurora B to guide end-on capture of spindle microtubules via phase separation

Chromosome segregation in mitosis is orchestrated by the dynamic interactions between the kinetochore and spindle microtubules. Our recent studies show that mitotic motor CENP-E cooperates with SKAP and forms a link between kinetochore core MIS13 complex and spindle microtubule plus-ends to achieve accurate chromosome alignment in mitosis. However, it remains elusive how SKAP regulates kinetochore attachment from lateral association to end-on attachment during metaphase alignment. Here, we identify a novel interaction between Aurora B and SKAP that orchestrates accurate interaction between the kinetochore and dynamic spindle microtubules. Interestingly, SKAP spontaneously phase-separates in vitro via weak, multivalent interactions into droplets with fast internal dynamics. SKAP and Aurora B form heterogeneous coacervates in vitro, which recapitulate the dynamics and behavior of SKAP comets in vivo. Importantly, SKAP interaction with Aurora B via phase separation is essential for accurate chromosome segregation and alignment. Based on those findings, we reason that SKAP–Aurora B interaction via phase separation constitutes a dynamic pool of Aurora B activity during the lateral to end-on conversion of kinetochore–microtubule attachments to achieve faithful cell division.

Computational Screening of Phase-separating Proteins

Phase separation is an important mechanism that mediates the compartmentalization of proteins in cells. Proteins that can undergo phase separation in cells share certain typical sequence features, like intrinsically disordered regions (IDRs) and multiple modular domains. Sequence-based analysis tools are commonly used in the screening of these proteins. However, current phase separation predictors are mostly designed for IDR-containing proteins, thus inevitably overlook the phase-separating proteins with relatively low IDR content. Features other than amino acid sequence could provide crucial information for identifying possible phase-separating proteins: protein–protein interaction (PPI) networks show multivalent interactions that underlie phase separation process; post-translational modifications (PTMs) are crucial in the regulation of phase separation behavior; spherical structures revealed in immunofluorescence (IF)images indicate condensed droplets formed by phase-separating proteins, distinguishing these proteins from non-phase-separating proteins. Here, we summarize the sequence-based tools for predicting phase-separating proteins and highlight the importance of incorporating PPIs, PTMs, and IF images into phase separation prediction in future studies.

Assembling the right type of switch: Protein condensation to signal cell death

Protein phase transitions are particularly amenable for cell signalling as these highly cooperative processes allow cells to make binary decisions in response to relatively small intracellular changes. The different processes of condensate formation and the distinct material properties of the resulting condensates provide a dictionary to modulate a range of decisions on cell fate. We argue that, on the one hand, the reversibility of liquid demixing offers a chance to arrest cell growth under specific circumstances. On the other hand, the transition to amyloids is better suited for terminal decisions such as those leading to apoptosis and necrosis. Here, we review recent examples of both scenarios, highlighting how mutations in signalling proteins affect the formation of biomolecular condensates with drastic effects on cell survival.

PLK4 is a microtubule-associated protein that self-assembles promoting de novo MTOC formation

The centrosome is an important microtubule-organising centre (MTOC) in animal cells. It consists of two barrel-shaped structures, the centrioles, surrounded by the pericentriolar material (PCM), which nucleates microtubules. Centrosomes can form close to an existing structure (canonical duplication) or de novo. How centrosomes form de novo is not known. The master driver of centrosome biogenesis, PLK4, is critical for the recruitment of several centriole components. Here, we investigate the beginning of centrosome biogenesis, taking advantage of Xenopus egg extracts, where PLK4 can induce de novo MTOC formation ( Eckerdt et al., 2011; Zitouni et al., 2016). Surprisingly, we observe that in vitro, PLK4 can self-assemble into condensates that recruit α- and β-tubulins. In Xenopus extracts, PLK4 assemblies additionally recruit STIL, a substrate of PLK4, and the microtubule nucleator γ-tubulin, forming acentriolar MTOCs de novo. The assembly of these robust microtubule asters is independent of dynein, similar to what is found for centrosomes. We suggest a new mechanism of action for PLK4, where it forms a self-organising catalytic scaffold that recruits centriole components, PCM factors and α- and β-tubulins, leading to MTOC formation.

This article has an associated First Person interview with the first author of the paper.

Summary: PLK4 binds to microtubules and self-assembles into condensates that recruit tubulin and trigger de novo microtubule-organising centre formation in vitro.